1. Field of the Invention
This invention relates to a DC electric power cable.
2. Prior Art
Polyethylene and cross-linked polyethylene have heretofore been extensively used as an insulation for a DC power cable since they are superior in dielectric strength and other dielectric characteristics. However, in the case where such a cable having an insulation of either polyethylene or cross-linked polyethylene is used as a high-voltage power transmission line, some difficulties are encountered. The most serious of these is that when the cable is supplied with a high DC voltage, long-lasting space charge tends to be produced in the insulation. It is generally considered that this space charge has the nature of electron, hole or ion and that such space charge results from the charge entrapped in those portions of the polyethylene insulation contributing to the formation of a crystal structure thereof. Polyethylene has good insulating properties and is non-polar, so that the entrapped charge can not easily be escaped from said those portions to form the long-lasting space charge. When such space charge is accumulated in the insulation of polyethylene upon application of DC voltage to the cable, the electric field intensity is increased adjacent to the conductor of the cable, so that the breakdown voltage of the cable is disadvantageously lowered.